The present invention relates to a method for monitoring an exhaust system of a motor vehicle having an internal combustion engine and having monitoring electronics.
German patent publication DE 100 13 893 A1 has disclosed a method for monitoring an exhaust system of a motor vehicle having an internal combustion engine. In this method, the catalytic activity of a catalytic converter arranged as a component with a purifying activity in an exhaust pipe section is assessed. The catalytic activity is assessed by determining the light-off temperature of the carbon monoxide oxidation reaction. The carbon monoxide oxidation reaction process is recorded by corresponding sensors arranged upstream and downstream of the catalytic converter. In addition, the exhaust-gas temperature downstream of the catalytic converter is measured, for which purpose a temperature sensor is arranged at the outlet side of the exhaust pipe section which is intended to accommodate the catalytic converter. Monitoring electronics determine the difference between the exhaust-gas temperature downstream of the catalytic converter and the light-off temperature. The activity of the catalytic converter is assessed on the basis of this result and of the carbon monoxide conversion rate recorded by sensor means, and the exhaust system is monitored in this way.
Patent EP 0 442 648 A2 has disclosed a method for monitoring a catalytic converter, in which the exhaust-gas inlet temperature and the exhaust-gas outlet temperature of the catalytic converter are measured. The measured temperature values are evaluated by forming the difference between them, and this difference is then subjected to an integration operation. The integration operation smooths the temperature curve or the value curves, thereby avoiding misinterpretations.
U.S. Pat. No. 5,560,200 A has disclosed a method for monitoring a catalytic converter, in which the temperature of the support structure of the converter or the temperature of the coating applied to it is determined at at least one location. Furthermore, the exhaust-gas temperature is determined upstream of this location, and a time derivative of the temperatures and the difference between the time derivatives are formed. A change in the sign of this difference is interpreted as the catalytic activity of the catalytic converter starting, so that the light-off of the catalytic converter is detected.
Patent EP 0 756 071 A2 has disclosed an apparatus for determining a deterioration in a catalytic converter arranged in an exhaust system. The apparatus comprises a temperature sensor for measuring the catalytic converter temperature and a control unit which gives an estimated value for the catalytic converter temperature. These may be temperatures at the downstream end of the catalytic converter. A conclusion is drawn as to the state of aging of the catalytic converter from the relationship between the measured and estimated catalytic converter temperatures.
By contrast, it is an object of the present invention to provide a method which allows more general monitoring of an exhaust system.
According to the present invention, this object is achieved by a method described and claimed below.
The method according to the invention is distinguished by the fact that the monitoring electronics compare a time curve of the outlet-side exhaust-gas temperature T2 with a time curve of an inlet-side exhaust-gas temperature T1 at the inlet side of the exhaust pipe section and/or with a time curve of a calculated value T2* for the exhaust-gas temperature at the outlet side of the exhaust pipe section. The calculated value T2* is determined on the basis of the heat-storing and/or fluid-dynamic action of the component with a purifying activity.
This procedure makes advantageous use of the phenomenon whereby a component with a purifying activity which is installed in an exhaust pipe section influences the exhaust-gas temperature and its time curve. In this context, a component with a purifying activity will primarily be a particulate filter or an exhaust-gas catalytic converter which influences the exhaust-gas temperature even without the occurrence of reaction exothermicities, on account of its heat-storing action. However, by way of example a switchable cooling circuit or a component which acts passively in some other way, preferably in heat terms, may also be considered as a component with a purifying activity. If heat-storing effects do not occur, by way of example it can be concluded from this that the component is absent. Therefore, if the curves for the temperatures at the inlet side and at the outlet side of the exhaust pipe section are determined and compared with one another in a suitable way, it is possible to assess whether a component with a purifying activity has been installed in this exhaust pipe section. Furthermore, the method according to the invention, by suitable comparison of the temperature curves, makes it possible to indicate any behavior which is unusual in this respect whereby an incorrect component is arranged in the exhaust pipe section. Likewise, the method according to the invention also allows the detection of leaks in the pipe section located between the inlet side and the outlet side, on account of the fluid-dynamic effect of a leak. If irregularities are determined during the monitoring of the exhaust system, it is, of course, possible to provide information to that effect in any desired way, for example in the form of a warning signal.
The comparison referred to may be between a curve of the outlet-side temperature T2 and a curve of an inlet-side temperature T1. However, it is also possible for the curve of the outlet-side temperature T2 to be compared with a curve of a temperature T2* which is to be expected on the outlet side. The latter is preferably determined with the assistance of models or characteristic diagrams, taking account of thermal and fluid-dynamic aspects and also taking account of the expected behavior of the component with a purifying activity and the current operating state of the motor vehicle. In a similar way, it is also possible, of course, to determine the inlet-side temperature T1 and its curve by calculation or with the aid of characteristic diagrams. By contrast, the outlet-side temperature T2 is measured directly by means of a suitable measurement sensor on the outlet side of the exhaust pipe section, i.e. within the cross-sectional area which delimits the exhaust pipe section on the outlet side. It is preferable for the time sections in which the comparison of the temperature curves is evaluated to be selected taking additional criteria into account.
In one configuration of the method, the time derivatives dT1/dt and dT2/dt of the inlet-side temperature T1 and the outlet-side temperature T2 are determined, and the difference dT1/dt−dT2/dt between the derivatives is determined and the result assessed. By forming the time derivatives of the temperatures, it is possible to particularly successfully characterize the curve of the temperatures. The formation of the difference, on the other hand, is particularly suitable for a comparison.
In a further configuration of the method, the monitoring electronics generate a signal which indicates the absence of the component with a purifying activity or the presence of an incorrect component if the difference dT1/dt−dT2/dt between the derivatives is within a predetermined range of values. The basis for this configuration is formed by the discovery that a component with a purifying activity in many operating situations manifests itself as a heat sink or as a heat source. Primarily on account of its heat capacity action, the presence of a component with a purifying activity manifests itself through a greater or lesser, positive or negative difference dT1/dt−dT2/dt. If this difference is not observed to a sufficient extent, i.e. if the difference dT1/dt−dT2/dt is within a range of values which is predetermined by two limit values and is preferably relatively small, around zero, it is possible to conclude that a component with a purifying activity is not present in the exhaust pipe section. This is then indicated by the generation and outputting of a corresponding signal. Analogously to this, it is possible to interpret a corresponding deviation from the expected influence of a component with a purifying activity on the temperature curves as meaning that an incorrect component has been fitted, since the correct component would have resulted in a difference outside the predetermined range of values.
In a further configuration of the method, the monitoring electronics generate a signal which indicates the absence of the component with a purifying activity or the presence of an incorrect component if the difference dT1/dt−dT2/dt between the derivatives is within a predetermined range of values and the time derivative dT1/dt of the inlet-side temperature T1 is outside a predetermined range of values. The dynamics of the entry-side temperature curve are also taken into account by taking account of the rate of change dT1/dt of the temperature T1 at the inlet side of the exhaust pipe section when evaluating the difference dT1/dt−dT2/dt. Since with high dynamics of the entry-side temperature curve the heat-storing action of a component makes its presence particularly strongly felt in the exhaust pipe section, this also allows a particularly reliable assessment of the exhaust pipe section and therefore makes the conclusion which is drawn particularly reliable. Moreover, influences of exothermicities which may be generated by the component with a purifying activity are advantageously minimized by taking account of the rate of change dT1/dt of the temperature T1 at the inlet side of the exhaust pipe section.
In a further configuration of the method, the monitoring electronics determine the time derivatives dT2/dt and dT2*/dt of the outlet-side exhaust-gas temperature T2 and of the calculated value T2* and determine the difference dT2*/dt−dT2/dt between the derivatives. Accordingly, the rate of change of the temperature T2 at the outlet side of the exhaust pipe section and the rate of change of the temperature T2* to be expected there are determined, and the curve of the temperatures is recorded in this way. The comparison is carried out by forming the difference. It is in this way likewise possible to assess whether a component with a purifying activity is present in the exhaust pipe section or whether an incorrect component is arranged there.
In a further configuration of the method, the monitoring electronics generate a signal which indicates the absence of the component with a purifying activity or the presence of an incorrect component if the difference dT2*/dt−dT2/dt between the derivatives is outside a predetermined range of values. This is possible since if a component with a purifying activity is present, the temperature T2 measured at the outlet side of the exhaust pipe section should correspond to the temperature T2* which is to be expected there and has been determined by calculation. This means that if the expected component is present, a value for the difference dT2*/dt−dT2/dt can be expected to lie within a range of values given by two predeterminable limit values. If this is not the case, it can be assumed that the component which is intended to be provided in the exhaust pipe section is not in fact present.
In a further configuration of the method, the monitoring electronics determine the time derivative dT1/dt of the inlet-side exhaust-gas temperature T1 and generate a signal which indicates the absence of the component with a purifying activity or the presence of an incorrect component if the difference dT2*/dt−dT2/dt between the derivatives is outside a predetermined range of values and the time derivative dT1/dt of the inlet-side temperature is outside a predetermined range of values. The additional inclusion of the rate of change dT1/dt of the inlet-side temperature T1 of the exhaust pipe section in the comparison has the advantage of improving the reliability of the decision which is made.